Path computation entities (PCE's) are widely used in dense-wavelength-division-multiplexed (DWDM) networks to determine how to route wavelength connections between node pairs. To compute the path, the PCE needs to know if two nodes are “reachable”—whether the DWDM network can support a connection between them with sufficient optical-signal-to-noise ratio (OSNR) to maintain good performance over the lifetime of the system. If the path is not “reachable” then additional resources may be utilized to reliably carry data between the two nodes.